The present invention relates to electrodeless light sources excited by high frequency power and, more specifically, to techniques and apparatus for optimizing the transfer of power from a source to the lamp at the time that the lamp is started.
Historically, there have been three methods of exciting discharges with electrodes. The first uses the discharge as a lossy part of either the capacitance or inductance of a tank circuit. A second method is to place the lamp in the path of radiation from a directional antenna. A third method uses a resonant cavity which contains the lamp and a device for matching the cavity input impedance to the source and transmission line. Examples of a device according to this third method may be found in "Microwave Discharge Cavities Operating at 2450 MHz" by F. C. Fehsenfeld et al., Review of Scientific Instruments, Volume 36, Number 3, (March, 1965). Another example of a resonant device is described in the U.S. Pat. No. 3,787,705 to Bolin.
All of these electrodeless light sources have disadvantages which prohibit their being seriously considered as a replacement for the conventional, electrode-containing incandescent light bulb. For example, one of the problems in the development of an electrodeless light source having potential commercial use resides in devising an effective method of starting the lamp. Electrodeless lamps prior to excitation and breakdown of the fill material have an extremely large impedance, whereas once the lamp is operating, the impedance is some lower value ranging up to a few hundred ohms. Thus, the light source should have a convenient, economical and practical way of compensating for the drastic change in load impedance.